Evidence has been accumulating that blood group antigens are modified in human epithelial tumors. The antigens of cell membranes have been identified as fucolipids and the changes have been observed as a deletion of normal blood group A, B, or Leb determinant and their replacement by precursor blood group antigens (H, Lea, and X-hapten, etc.). A glycolipid showing a reactivity with human carcinoembryonic antigen was isolated and showed a close chemical and immunologic relationship to the basic structure of blood group antigen. In some tumors, the appearance of a heterologous or incompatible blood group antigen (e.g. A-like antigen in B or O individuals) has been observed. This is to propose studies on: (1) further characterization of blood group glycolipids and glycoproteins of normal and tumor tissue; (2) structural and enzymatic bases for blood group changes of tumors in general, but focusing on H and i-activity changes; (3) comparison of the blood group changes occurring during oncogenesis and that occurring during ontogenesis; (4) characterization of A-like antigen appearing in tumors of host blood group O and B individuals; (5) structure of a variant component of blood group A glycolipids reacting with wheat germ lectin and with anti-carcinoembryonic antisera. For further systematic studies on changes of blood group and fucolipids, a model study on experimental gastrointestinal tumors is planned in addition to clinical and autopsy cases.